Applying Composting to Waste Management
Composting has an appeal to local authorities needing to meet diversion targets while keeping a watch on their budgets, since it is relatively simple and does not demand particularly high resource investment, either to set up or run. As a consequence, many of the initiatives instigated to deal with biowaste have been based on composting of one form or another. In the broadest of terms, such schemes fall into one of two categories, namely, home composting, or centralised facilities. The focus of this section will fall on the latter, as a more representative application of biotechnology, though to set this in context, it is worth giving a brief outline of the former.
Home-based systems differ little in reality from the traditional gardener’s approach, putting biodegradable material into a heap or, more typically a bin, often provided free or at a subsidised price, by the local council. Though this does have the advantage of directly involving people in the disposal of their own waste and the informality of this approach has its own advantages, such schemes are not without certain drawbacks. To work, these initiatives draw heavily on householder goodwill and competence, not to mention a good choice of bin and simply making the means available does not, of course, ensure that it will be used. Anecdotal evidence suggests that many bins lie unused within two years, once the initial enthusiasm wears off, and an investigation into Luton’s trial scheme suggests that home composting may make little difference to the overall amount of waste generated (Wright 1998). The kind of instant minimisation popularly supposed would seem to be far from guaranteed.
One clear advantage that household composters do have, however, is the ability to control very closely what goes into their system. This avoids both the issues of contamination and the need for post-user segregation typically foisted on the operators of centralised facilities. Thus, although domestic initiatives of this kind are unlikely ever to make the sort of difference to biowaste treatment demanded by legislation on their own, it seems likely that they will always have a role to play, perhaps most especially in remoter areas where collection for processing elsewhere might prove uneconomic.
The biochemistry and microbiology of all composting remains essentially the same, irrespective of the details of the operation. However, the scale of schemes set up to deal with a municipal biowaste stream in terms of the physical volume involved imposes certain additional considerations, not least amongst them being the need to ensure adequate aeration. In the back-garden compost heap, oxygen diffuses directly into the material; large-scale composting cannot rely on this method, as the large quantities involved lead to a lower surface area to volume ratio, limiting natural oxygen ingress. To overcome this, various techniques make use of mixing, turning or pumping, but, clearly, the additional energy required has its own implications for a commercial operation.
Approaches suitable for municipal scale use fall into five main categories:
A sixth form, termed ‘tower composting’, may occasionally be encountered, but it is generally much less common than the other five.
No one system is the universal ideal. The decision as to which approach is likely to be the most suitable for specific conditions is dependent on a number of factors, including the nature and quantity of biowaste available, the required quality of the end-product, the time available for processing, local workforce and land availability and financial considerations.
The biowaste is laid in parallel long rows, around two or three metres high and three or four metres across at the base, forming a characteristically trapezoid shape. Windrowing is usually done on a large scale and, though they can be situ-ated under cover, generally they tend to be outdoor facilities, which exposes them more to the vagaries of the weather and makes process control more difficult. While this might be a problem for some kinds of biowaste, for the typical park and garden waste treated by this method, it generally is not. However, some early attempts were prone to heavy leachate production in conditions of high rainfall, leading to concerns regarding localised soil pollution. This was largely an engi-neering problem, however, and the almost universal requirement for a suitably constructed concrete pad and interceptor has made this virtually unknown today.
Limited aeration occurs naturally via diffusion and convection currents, but this is heavily augmented by a regime of regular turning, which also helps to mix the composting material, thus helping to make the rate of breakdown more uniform. Dependent on the size of the operation, this may be done by anything from front-end loaders on very small sites, to self-propelled specialised turners which straddle the windrows at larger facilities. The intervals between turning can be tailored to the stage of the process, being more frequent early on, when oxygen demand is high, becoming longer as composting proceeds.
Windrows have a typically high land requirement, can potentially give rise to odour problems and are potentially likely to release fungal spores and other bioaerosols during turning. Despite these drawbacks, this approach accounts for the vast majority of centralised composting projects, possibly because it is often carried out as an addition to existing landfill operations, thereby significantly reducing the actual nuisance generated.
Superficially resembling the previous method, the static pile, as its name suggests, is not turned and thus does not have to conform to the dimensions of a turner, allowing the rows to be considerably taller and wider. What mixing is needed can be achieved using standard agricultural equipment and so these systems tend to be significantly cheaper in respect of equipment, manpower and running costs. They do not, however, remove the land requirement, since decomposition progresses at a slower rate, causing the material to remain on site for a longer period.
In an attempt to get around this, a variant on the idea has been developed, particularly for the co-composting of food or garden biowaste with manure or sewage sludge, which relies on forced aeration. With a perforated floor and fans to push air through the material, the characteristically low oxygen level within the core of traditional static piles is avoided and processing accelerated. However, bulk air movement is expensive, so this system tends to be reserved for small tonnage facilities, often in areas where good odour control is of major importance.
Tunnel composting has been used by the mushroom industry for a number of years, where processing takes place inside closed tunnels, around five metres high and up to 40 feet in length. There has been some interest in adapting it to deal with MSW-derived material and one system which has evolved uses huge polythene bags, a metre or so high and 60-metres long into which a special filling machine packs around 75 tonnes of source separated putrescible material. This particular design also makes use of a fan to force air through the material rather like the previous technique, with slits in the side wall allowing carbon dioxide to escape.
The processing time is reduced compared with a similar sized aerated static pile, since the environmental conditions within the tunnel are easier to control, though similar cost considerations apply.
Rotary drums seem to drift in and out of fashion, often being favoured by those needing to co-compost sewage sludge with more fibrous material, like crop residues, straw or garden waste. The principle is simple; the waste is loaded into the drum which then slowly rotates. This gently tumbles the material, mixing it and helping to aerate it. The drums themselves are usually steel, insulated to reduce heat loss.
Sometimes also called closed reactor composters, there are a number of designs of in-vessel systems available, ranging from small steel or plastic tanks, through larger metal cages to long concrete troughs with high sidewalls. The main char-acteristic of these systems is that the waste breaks down within an enclosed container, which allows the internal environmental conditions to be closely con-trolled. This approach offers a very efficient use of space and close regulation of the process, since some form of mechanical aeration is also required it is signifi-cantly more expensive on a tonne for tonne basis than the less resource-intensive methods. Accordingly, it is less suitable for large capacity requirements, it has a role in smaller scale operations or where the material to be treated does not easily fit into other kinds of processing or disposal arrangements.
This is less of a natural group than the preceding approaches to composting, since it encompasses far greater variety of design. Consequently there is a marked variance in the capacity, complexity and cost of these systems.
Aside from aeration, which has already been discussed, a number of other param-eters affect the composting process. Although these are themselves influenced to some extent by the method being used, in general the most important of these factors are:
•nature of the feedstock;
The temperature changes over the stages of composting have important implica-tions for the efficiency of the process. It is widely agreed that for satisfactory sanitisation, the material should reach at least 55 ◦ C, though there is less of a consensus over the required duration of this exposure.
On the other hand, the temperature should not be permitted to exceed 70 ◦ C, since above this most of the compost microbes either die off or become inac-tivated, causing the biological breakdown to slow or stop. In a commercial operation, lost processing time has inevitable financial consequences.
A moisture content of around 60% is the ideal target for optimum composting, though anything within a range of between 40 – 70% will suffice. While some biowastes meet this requirement naturally; others forms can be surprisingly dry, sometimes with a moisture content as low as 25 – 30%, which approaches the levels at which severe biological inhibition can occur. Equally, too wet a material may be a problem as this may restrict aeration and even encourage leaching. Even when the initial mix is right, composting matter gradually loses moisture over time and evaporative losses from the surface of the composting biowaste can cause problems, especially in frequently turned windrow regimes. Careful monitoring and appropriate management is necessary to ensure that the optimum range is maintained.
The optimum particle size for composting is, of necessity, something of a com-promise. The smaller the individual pieces, the larger the surface area to volume ratio, which makes more of the material available to microbial attack, thus speed-ing up the process of decomposition. However, particles which are shredded too finely will tend to become compacted and so reduce aeration within the material. Consequently, a balance must be struck, providing the smallest possible particle size which does not interfere with air flow. Individual design features may need to be considered; dependent on the system used, bed depth, aeration method and the nature of the biowaste itself can all have an influence.
The importance of the carbon to nitrogen (C:N) ratio and the need for careful management to ensure a proper balance has already been discussed. In addition, for some materials, the use of amendments or co-composting with other wastes can also help optimise conditions for biological treatment. Sewage sludge and manures are often used in this way, but they can also boost the available nutrient levels, often in an uncertain way and by variable amounts. Generally, additives are used where there is a need to improve either the chemical or the physical nature of the composting material. Clearly, for a large commercial operation, it is essential that whatever is used does not significantly affect the economics of the plant and for this reason, although artificial fertilisers are an ideal way of increasing nutrient content, they are seldom used in household waste applications. Their expense relative to these low-cost biowastes effectively rules them out; they are, however, often to be seen in ex situ bioremediation operations, since composting contaminated soil commands a higher price
Additions to the original material typically accelerate processing, but careful monitoring is essential since the blend may exhibit very different decompo-sitional characteristics, which may ultimately influence the nature of the final product derived.
Although gardeners have a number of proprietary brands of compost accelerants available to them, this is not an approach often used at commercial facilities, mainly due to the scale of these operations and the consequent expense. As with nutrient addition, this tends to be reserved for use on high value wastes, though many common substances used in co-composting programmes, like manures, are themselves widely accepted to act as natural accelerants. Though their effect is more variable, it seems likely that this is the only form of enhanced processing applicable for general biowaste use.
In many respects, the time required is a function of all the other factors. Process-ing garden or food waste can be achieved in under three months using aerated, in-vessel or turned windrow systems, while in a simple static pile, it may take a year or more to reach the same state. Inevitably much depends also on the man-agement regime, since process optimisation is the key to accelerated biotreatment and good operation practice is, consequently, of considerable importance.